I am working on a half bridge isolated SMPS to deliver 13.8V at around 30A or more. The power supply is for amateur radio use and not audio, but browsing through some previous posts there seems to be plenty of expertise on this forum.
I intend to use power Mosfets as the switching elements with transformer gate drive, although if I am convinced otherwise I may have the controller on the primary side and use a high/low side driver by IC or discrete components. Before I commit to using Mosfets however, I do have plenty of ex TV horizontal output transistors (BU208, BU508 etc.) which may be up to the job. Would like some advice to help with this decision.
I have been scrounging around in my junk box and have found a very nice core and bobbin, the core is an E-E type, each half measures 26mm(W) by 32mm(H) by 65mm(L). This core must be far bigger than my requirements, but I assume it wont hurt to use too big a core. It is made of N27 material, I have so far failed to find a datasheet for the core so would like advice on its frequency range, power handling capacity and how to calculate how many turns I need on the primary. Seeing as it is so big, I wonder if I can run it at a fairly low frequency, say 25kHz, to reduce switching losses and reduce EMC problems. I know its a bulky core for the job, but still very much smaller and lighter than a 50Hz lump.
I reckon current mode control would be best so that the control pulses can be rapidly terminated in the event of a short circuit. I have ordered some SG3525 chips.
Regarding the relatively low output voltage I am considering synchronous rectification to further reduce losses.
I live in the UK and my mains supply is 240V 50Hz.

I have seen several designs where PC power supplies are modified to slightly increase the output voltage, some examples just change a couple of components in the feedback section, others involve rewinding the transformer and are more involved. I certainly have a good collection of parts from scrap PC power supplies and have been experimenting with them, I have found that if the switching transistors blow a couple of TV horizontal output transistor can be fitted as a straight swap and work fine. The problem I have with PC power supplies is that the feedback circuit is over complicated since there is also 5V and sometimes 3.3V to consider, the rectifiers on the 12V side are very underrated and I think the claims for the output power of these units are along the same lines as the quoted ratings for the output of computer speakers; i.e. exaggerated and basically lies. The more modern PC power supplies seem to be increasingly of the flyback type.
My main aim of this project is to build a supply from scratch and modifying a PC supply, while probably practical, will not give me the satisfaction of being able to say I did it myself.

I have tried the links to the N27 datasheet but could not open them, I was informed 'the file is damaged and could not be opened'. I did find a datasheet for this material elsewhere and found the Bmax value to be 500mT. With this information, with the voltage and effective x-sectional area of the core I should be able to work out how many turns I need. My question now is, how do I calculate this area? do I simply add up the area of the faces of the centre limb and the two side pieces, or just the centre limb, or is it more involved? I may post a photo of the core and perhaps someone can estimate the power capability.

The second link gives you almost all the information you need to build from the scratch a half-bridge smps. If you browsing some Unitrode seminars (google it) you will find information to wind the transformer and the inductors that wil be needed.

I stumbled across the information last night whilst browsing the web, the effective core area is about 90% of the measured area of the centre limb, and I know the Bmax value to be 500mT so I can get on with calculating how many turns I need. When I have wound the primary, I may wind the secondary to give about 240V and use some domestic 100W light bulbs for testing purposes, and just check all is well before working on the DC side of things. I have already tested the half bridge this way and the switching transistors run cool with 150W worth of resistive load. If I place a small resistor in series with this load, and couple my oscilloscope across it (dont worry im using an isolation transformer) will I be able to tell from the current waveform if the core is getting near saturation?

You can't use N27 at 25 KHz and 500 mT, because the core losses will be huge. Check the material data sheet for more information. They should give information that shows core losses vs B and f.

Ok, just checked the N27 datasheet and it says its optimum frequency range is 25 to 150kHz, so ok on that score. The Bmax is quoted as 500mT at 10kHz. Is the Bmax value frequency dependant then? do I use a lower Bmax at higher frequencies?
Further down the datasheet it quotes core losses as 155kW/m^3 at 25kHz and 200mT. What Bmax value should I be using for 25kHz, and how is it related to the transformer frequency?

I have given up on the idea of using 25kHz as the transformer frequency now, I have just tried it out (calculated 38 turns for Bmax=200mT, DC input to the half bridge is 325V on load, does this sound reasonable?) and although it seemed to work fine, the transformer sounded dreadful; I think 25kHz is outside the range of human hearing, so perhaps its the 12.5kHz pulses that I can hear. The bobbin is a fairly loose fit on the core, I dont know if a blob of epoxy would cure this problem. I changed the timing resistor (im using a TL494 for now, manually controlling the pulse width) and moved the frequency to 35kHz using the same number of turns as before, and its silent. Is there a problem if too many turns are used? it didn't seem worth rewinding for a relatively small change in frequency, I expect the Bmax value will just be a little lower. I will post some photos of my work so far once I have charged the camera battery.